Experts have suggested that the cause of the break-up was probably mechanical or structural.

Heightened security had surrounded Columbia's latest mission because of the presence of Colonel Ilan Ramon, the first Israeli in space.

There was a shocked reaction in Israel to the disaster. "The people of Israel bow their heads today in view of this great tragedy," said a statement from Prime Minister Ariel Sharon's office.

Cargo launch

Russian president Vladimir Putin said his country's co-operation in space exploration made the accident even more tragic for Russians. But the scheduled launch of a Russian cargo vessel to serve the International Space Station (ISS) will go ahead on Sunday.

Hundreds of US state troopers, police and rescue workers are searching large areas in eastern Texas for debris from the shuttle.

Columbia, which had been due to land at 0916 EST (1416 GMT) was returning from a 16-day mission orbiting the Earth and had just begun its re-entry procedure when contact was lost at about 0900 EST.

Nasa said the shuttle was about 200,000 feet up and travelling at 12,500 mph (20,000 km/h) at the time.

Television pictures showed a vapour trail from the craft as it flew over Dallas.

It then appeared to disintegrate into several separate vapour trails, and witnesses in the area said they heard "big bangs".

Texas public safety department spokesman Clive Kennelly said there were more than 2,000 debris fields, scattered from the small town of Nacogdoches about 170 miles (290 km) south-east of Dallas, to the Louisiana border.

Nasa has warned that any debris found should be avoided as it could be hazardous, and that people should report such finds to the authorities.

Pieces of debris have been reported in fields and on roads, and one Nacogdoches resident, Jeff Hancock, said a metal bracket about a foot (30 centimetres) long crashed through his office roof, the Associated Press news agency reported.

Support for families

Never before in 42 years of human spaceflight, has Nasa lost a space crew during landing.